Filled telecommunications cable having temperature stable mutual capacitance

Abstract: A cable having a thixotropic filling compound therein, the filling compound being a gel composition including a water absorbent hydrocarbon polymer with pendent anionic groups. The composition eliminates shorts caused by moisture contact with such wires. The cable including the composition is stabilized against thermally induced capacitance changes and shows improved oxidation induction times.

Description: THIS INVENTION relates to a telecommunications cable, and a method of making such a cable. Communications cables such as telephone lines are typically copper wire, which are insulated from each other with a thick layer of a thermoplastic resin and bundled by an insulating material. Bundles of insulated pairs of conducting wires are then wrapped with a sheath of plastic, paper wrapping or other material, into a cable. A filler such as a petroleum gel is added to many cables inside the cable cover to fill the interstitial spaces and retard water migration therein. In recognition of the essential nature of the ability of telecommunications cable to be able to withstand exposure to water, the industry has promulgated certain performance standards which the cable must achieve. In particular, one standard requires that a 91.5 cm (three foot) long section of cable not pass water when maintained under a column of water 91.5 cm (three feet) high (e.g. a "three foot water head"). The industry is currently considering a change in that standard to require that a section of cable 2.44 metre (eight feet) long not pass water when subjected to 3.66 metre (twelve feet) of water head for twenty four hours. The prior art discloses protection of telecommunications cable against water invasion by filling the spaces between the wrapped bundles of conducting wires inside the cable (referred to as the filling zone) with compounds such as polyethylene-petroleum jelly (PEPJ) and oil-extended thermoplastic rubbers (ETPR), the latter being widely used by AT&T and the Bell regional operating companies in the United States and sold under the trademark FLEX-GEL. For example, US-A-4,259,540 discloses a suitable oil-extended thermoplastic rubber for use in a cable. Certain Patents also describe telecommunications cables including water swellable polymers such as polyvinyl alcohol, polyacrylamides, or cellulose derivatives, which are applied to bundle wrappings or contained in "moisture barriers" which are spaced internally along the length of the cable. For example, US-A-3,607,487 and US-A-3,717,716 discloses the use of materials comprising petroleum jelly mixed with a polymer such as polypropylene as a filling composition for a cable. Such cables are, however, characterised by a number of limitations and disadvantages. It has been shown that the oils in PEPJs and ETPRs that are used in prior art cables are absorbed into the polyolefin insulation of the paired conductors in the cable. This absorption causes the insulation to swell, causing increased spacing between conductors and thereby decreasing the mutual capacitance within the cable. In addition, it has been shown that in polyolefin foam insulation, continued oil absorption causes the air spaces within the foam (cells) to become partially filled with the oil. As the cells fill with oil, the effective dielectric constant of the foam is increased, thereby increasing the mutual capacitance between the paired conductors, which affects the impedance of the cable. In addition, changes in the capacitance may affect crosstalk characteristics of the cable. The term "effective dielectric constant" refers to the composite effects of the various dielectric constants of the materials between the conductors; for example, air has a dielectric constant of one and polyethylene has a dielectric constant of about 2.2 and foamed insulation is made up of these two components and therefore has a dielectric constant between that of the two components depending upon the ratio of polyethylene to air. As the air in the cells is displaced by absorbed oil, displacing air with material having a higher dielectric constant, the effective dielectric constant is increased. The capacitance of the cable is also affected by temperature. As temperature increases, the materials within the cable undergo thermal expansion, causing increased spacing between conductors and resulting in a decrease in capacitance. With known commercially available insulations and filling compounds, the dielectric constants of the materials are essentially constant over the operating temperature range of the cable. Therefore, in currently commercially available cables, capacitance change is a function of thermal expansion. Changes in capacitance due to temperature changes occur immediately with temperature change of the cable whereas capacitance changes due to oil absorption and/or cell filling are gradual over time (weeks to years), the rate depending upon the average temperature to which the cable is exposed. Changes due to absorption are irreversible whereas changes in capacitance due to temperature changes are cyclical as ambient temperature rises and falls. ETPR filling compounds are also characterised by a number of disadvantages and/or limitations. For instance, ETPRs must be heated to achieve a liquid state for handling and filling of the cable, increasing cost and creating logistical problems during storage and transport of the material. Cable is filled at about 110°C (230°F) and under pressure such that the filling operation is relatively dangerous and thermal contraction after filling results in the formation of voids which can serve as paths for water migration. Further, only recently have ETPRs been available which can be used in aerial cables; the temperatures in the cable resulting from ambient temperature and exposure to sunlight caused previous ETRPs to liquify and drip out of the cable. US-A-4,870,117 is directed to a filling compound which is stated to maintain its gel state at temperatures up to 80°C (e.g. the temperature to which aerial cables may be subjected), but reports from the field indicate that such cables may not be performing as expected. Further, so far as is known, cable including this material cannot meet the proposed 3.66 metre (12 feet) 24 hour industry standard for resistance to water penetration. Another problem with the use of ETPRs which has recently come to light is the cracking of the foam-skin polyethylene used to insulate the wires of the ETPR-filled cable. See , for instance, T.N. Bowmer, "Cracking of Foam-Skin Polyethylene Insulation in Pedestals", Proceed. 37th Int. Wire & Cable Symp. 475 (1988). The response of the industry to this problem was to increase anti-oxidation stabiliser content to obtain improved foam-skin life expectancy. That approach, however, does not address the fundamental issue of the compatibility (or incompatibility) of the resin, stabiliser(s) and/or filling compounds. Petroleum gels are generally used as filling compounds, in part because all known substitutes suffer from one or more disadvantages which limit their utility such that petroleum gels represent the least expensive alternative. EP-A-0,586,158 discloses a cable which is filled with a filling composition comprising a styrene-rubber block co-polymer and a super-absorbent polymer material. EP-A-0,541,007 discloses a filling compound for a cable comprising a mineral oil and a stellate substance. In spite of a continuing and long-felt need, and in spite of the many attempts which have been made to solve these problems, there is still a need for a filled cable having stable capacitance relative to temperature change. The ideal cable would maintain constant electrical parameters regardless of the temperature. According to one aspect of this invention there is provided a telecommunications cable comprising a plurality of paired conductors, insulation surrounding said paired conductors, and a wrapping round the insulated, paired conductors, the space therebetween being filled with a composition characterised in that the cable has a substantially stable mutual capacitance between said paired conductors as the cable undergoes thermal expansion, said composition being comprised of a dielectric base oil, a water absorbing polymer, and a thixotrope, wherein the said components are selected such that the composition has a dielectric constant which increases as the temperature to which the cable is exposed increases, thereby mitigating the decrease in the mutual capacitance of the cable resulting from the expansion effect of increased temperature on the paired conductors, insulation and wrapping. Preferably the composition is formed by mixing about 70 to 85 weight percent dielectric base oil, about 1 to 15 weight percent thixotrope, and about 6 to 20 weight percent water absorbing polymer. Advantageously the dielectric base oil is comprised of a hydrocarbon oil having a molecular weight of about 200 or higher and an extender. Conveniently the extender is a block co-polymer, thermoplastic rubber, polyethylene powder, microcrystalline wax, or polyethylene or ethylene microspheres, or mixtures of same. According to another aspect of this invention there is provided a method of making a telecommunications cable comprising the steps of wrapping a plurality of paired, insulated conductors into a bundle, and filling the spaces between the paired, insulated conductors in the bundle with a composition wherein the composition is comprised of a dielectric base oil, a water absorbing polymer, and a thixotrope, wherein the said components are selected such that the composition has a dielectric constant which increases as the temperature to which the cable is exposed increases, thereby mitigating the decrease in the mutual capacitance of the cable resulting from the expansion effect of increased temperature on the paired conductors, insulation and wrapping. Preferably a plurality of bundles of paired, insulated conductors are bound together with a sheath, and the spaces inside the sheath and between the bundles are also filled with the composition. In order that the invention may be more readily understood the invention will now be described, by way of example, with reference to the accompanying drawings in which: FIGURE 1 is a graph representing the capacitance of compositions as a function of time, and FIGURE 2 is a graph representing the capacitance of cables as a function of temperature. The telecommunications cable of the present invention includes a water absorbent thixotropic gel composition which provides an initial barrier to the entry of water into the area of the cable in which the composition is located. If water does enter the space, the water absorbent polymer in the gel is activated and the water is absorbed. In tests, water was placed adjacent to the gel composition. The fine powder-like polymer in the dielectric gel matrix is seen travelling to the water within the gel matrix. This effect appears to be the result of the water absorbent polymer seeking out the water. Once the water contacts the dielectric gel, a highly viscous semi-solid material is formed by migration of the polymer from out of the gel that, depending on the distance from the dielectric gel composition, is incapable of fluid movement and which restricts movement of the water to extremely slow diffusion-controlled movement. The water absorbent polymers, having pendent anionic groups, when exposed to a wire that is in short caused by the presence of water, causes an attraction of the anionic groups of the polymer to the wire, the wires acting, in effect, as an anode. This apparent attraction of the polymer to the exposed wire brings the polymer into electrochemical association with the wire, and the accumulated polymer that develops around the exposed wire excludes water from the surface of the wire. As that layer builds up around the wire, the short is attenuated and full flow of current through the wire is re-established with the result that the short is eliminated or "healed". Depending on the components of the gel and size of the flaw in the insulation, the healing process can take as short a time as several minutes up to about 2 or 3 hours after which current is re-established through the wire. The gel filling the described cable of the present invention therefore plays several roles in protecting the electrical conductors of a telephone cable from moisture damage. First, if there is invasive moisture, the gel composition repels the water. Additionally, in the presence of moisture, the water absorbent polymer of the dielectric gel is activated to travel out of the gel matrix to form a gel upon contact with the water. This travelling effect is particularly useful when the confined space is in a cable containing a multitude of wires in a bundle having very small interstitial spaces therebetween. The polymer travels into the interstitial spaces if moisture is present, thereby causing the water itself to participate in a plugging effect too prevent further invasion of water into the cable. In the case of, for instance, communication cables having small (pinhole-sized) flaws in the insulation around a conductor, an additional benefit is that any shorts which are present may be healed by the gel composition, restoring current through the conductor. The gel composition of the cable of the present invention, however, mitigates the change in capacitance due to temperature changes and also due to oil absorption by the insulation of the paired conductors of the cable. The oils used in formulating the composition filling the cable of the present invention are maintained in the thixotropic gel and therefore, are considerably less mobile and are not absorbed to the same extent as the oils of, for instance, PEPJs and ETPRs. Not only does the thixotrope maintain the oils in the gel, but preferably higher molecular oils are used to further decrease mobility. The thixotropic agents of the preferred gel composition of the cable of the present invention bind the oil into the gel and therefore do not depend exclusively on synthetic microcrystalline wax and/or rubber to prevent oil migration. Consequently, as the temperature to which the cable is exposed increases, less oil is free to migrate into the cells of the insulation. Because the thixotropic property of the gel allows the cable to be filled at ambient temperature (as compared to the increased temperatures required for ETPR-and PEPJ filled cables) there is no initial heat soak to begin migration of oil into insulation. The result of the reduced tendency of the gel composition which fills the cable of the present invention to migrate into the polyethylene insulation prevents the gel composition from imparting additional stress to the insulation as opposed to ETPR compounds. Lower stress levels within the insulation results in reduced cracking of the insulation over time. The decreased mobility of the oil component of the gel composition also decreases the cell-filling phenomenon and therefore mitigates capacitance change. An additional benefit of the decreased mobility of the oil component of the composition which fills the cable of the present invention is of particular benefit when the cable is used as aerial cable. As noted above, when PEPJ-and ETPR-filled cable cools after being filled at 110°C (230°F), small voids are left as a result of the varying degrees of thermal contraction of the components of the cable. Mobile oils in such filling compounds tend to accumulate in these voids, a process which is accelerated by the 80°C and higher internal temperatures which aerial cable can develop. Further, the oils tend to accumulate in the voids in the lower parts of the cable, a phenomenon which can actually vary the local concentrations of the various component ingredients of the cable, affecting the electricals and exacerbating the above-described aging/cell-filling processes. Not only does the decreased mobility of the oils of the filling compound which is utilised in the cable of the present invention help avoid these problems, but there are no thermal-induced voids in which the oil accumulates since the cable is filled at ambient temperature. The filled cable of the present invention is also characterised by another advantage which distinguishes the cable of the present invention over that filled with, for instance, ETPRs or PEPJs. The capacitance change due to changes in ambient temperature as a result of the temperature dependency of the dielectric constant of the filling compound is mitigated as a result of the inclusion of the water absorbing polymer. This temperature dependency results from a decrease in the spacing between conductors as temperature decreases. This reduction in space would be expected to cause an increase in capacitance in the case of cables filled with ETPRs or PEPJs. However, the gel composition which fills the cable of the present invention is engineered so that an increase in dielectric constant of the gel mitigates the effect of temperature. The polyolefin insulation and oil carrier of the filling compound have relatively constant dielectric constants; addition of the water absorbing polymer, which has a temperature-affected dielectric constant, has the result of maintaining an effective dielectric constant which is stable over the expected operating temperature range of the cable by off-setting, or mitigating, the changes in dielectric constant resulting from thermal expansion and/or contraction. A 25 pair, 24 gauge telecommunications cable manufactured for Applicant by an established, domestic cable manufacturer which includes the gel composition of the present invention has met and exceeded the above-described industry standards for cable performance. For instance, one section of such cable that is just 61 cm (two feet) in length has withstood about 15.25 m (fifty (50) feet) of water head (simulated with 158.56-172,34 kN/M 2 air pressure on water) for about twenty-four (24) months, and another section of such cable that is five feet long has withstood 18.30 m (sixty (60) feet) of water head for about twenty six (26) months without passing any water. Those experiments are on-going such that it is possible that this level of performance will continue for an indefinite, additional period of time. The water absorbent polymers which are suitable for use in connection with the filing compound of the cable of the present invention are those with a hydrocarbon chain backbone and pendent anionic groups on the hydrocarbon chain, and are preferably polymers of non-naturally occurring monomers so as to be less susceptible to bacterial degradation. The anionic groups can be carboxylate, sulfate, phosphate, sulfonate, phosphonate, or any other anionic groups which will form a negative charge on exposure to water, polycarboxylates being preferred. The preferred carboxylate polymers are those made from α,β-ethylenically unsaturated mono- and dicarboxylic acids and/or anhydrides such as propenoic acids, β-methylpropenoic acids, maleic acids, fumaric acids and the respective maleic and fumaric anhydrides. Particular success has been achieved using a polymer of 2-propenoate commonly referred to as polyacrylic, or propanoic, acid the anionic carboxylate groups of which, when exposed to aqueous conditions, yield a strongly negative charge along the polymer chain. The salt form of these polymers can be used with a variety of ions including, but not limited to, alkali metal ions such as lithium, sodium, potassium or alkali earth metals such as magnesium, calcium, strontium, barium, zinc or aluminium. The salt used will depend on the valency of the anionic group attached to the hydrocarbon chain backbone. Although the preferred water absorbent polymers are polycarboxylates, other super-absorbent polymers of acrylates, acrylamides, methacrylate, methacrylamide, acrylonitrile, methacrylonitrile, diacrylate, and starch graft polymers of those polymers (such as a starch-polyacrylonitrile graft polymer) may be used to advantage. Such polymers are resistant to biological degradation over a long period of time. Consequently, polymers of these non-naturally occurring monomers are collectively referred to as being "non-biodegradable" throughout this Specification. For instance, the polyacrylic acid polymer described above has been demonstrated to be resistant to degradation over a period of several years; controlled experiments with that polymer have shown no degradation for over one year. The water absorbent polymer is incorporated into the gel composition in concentrations ranging from about 5 to about 33% by weight of the final composition, depending upon the particular polymer utilised. Although satisfactory results have been obtained with compositions including concentrations of polymer at both ends of that range (hence the use of the word "about" in describing the range), concentrations of from about 6 to about 20% are preferred, and in the case of the preferred polyacrylic acid polymer, a concentration of from about 8 to about 15% is preferred. Selection of the particular polymer, and the specific proportion of the polymer that is selected, depends upon the desired degree of change of dielectric constant in the gel composition of the cable of the present invention. In other words,different polymers affect the change in dielectric constant resulting from changes in the temperature to which the cable is exposed, as do different proportions of the polymer. It is generally desired to select a polymer (and a proportion of that polymer) which, when incorporated into the gel composition, gives a dielectric constant for that composition which ranges between about 2.2 at 25°C up to about 2.3 at 65°C, but those skilled in the art who have the benefit of this disclosure will recognise that the type of polymer and/or its proportion may need to be modified for use in connection with, for instance, aerial cable which may reach temperatures of above 80°C. Selection of the particular polymer utilised, and the proportion of that polymer, is made by experimentation in accordance with the test described in Example 7, below. As the data set out in that Example illustrate, the preferred cable of the present invention is filled with a composition having a dielectric constant which carries by about -5.15% at -3°C up to about +5.69% at 60°C, using 25°C (and the approximate dielectric constant of 2.2) as the nominal "zero" point. A number of compositions which are gels or can be thickened to form a gel have been used as a gel matrix. The gel matrix must be relatively non-conductive to a small direct current, e.g. have a low dielectric constant. The matrix should provide a fairly uniform dispersal of the anionic hydrocarbon polymer in the gel. The viscosity and composition of the gel is varied depending on the method used to introduce the composition into the telecommunications cable and the temperature and conditions under which the cable is manufactured. The gel matrices used in this composition include silicones, petroleum gels, high viscosity esters, glycols, polyglycols, olefins and fluorocarbons. All such materials and/or mixtures are referred to collectively herein as dielectric oil gel matrices. Mixtures including napthenic and paraffinic oils are presently preferred for use as gel matrices in the composition of the present invention but those skilled in the art who have the benefit of this disclosure will recognise that any long chain, saturated oil is likewise used to advantage. Petroleum hydrocarbons must be free of impurities which could corrode the conductors in the cable. The gel matrix is used to advantage in concentrations ranging from about 40 to about 92% by weight. The preferred concentrations, depending on the particular material, range from about 70 to about 85% by weight. Thixotropes are used to achieve a desired viscosity. Suitable thixotropes includes those known in the art for thickening petroleum oils, fluorocarbons, waxes, petrolatums, gels and greases, and can include such materials as ethylene and polyethylene microspheres. Typical thixotropes for gels and greases are pyrogenic or fumed silica (e.g. CAB-O-SIL, Cabot Corp. and AEROSIL, Degussa Corp.), organophilic clays such as bentonite (e.g. BENTOLITE, Georgia Kaolin Co. and BENTONE and BARRAGEL, N.L. Industries/Rheox) and hectorite, soaps such as metal stearates, and ureas. The amount of the thixotrope which is utilised depends upon the viscosity desired, the particular gel matrix with which the thixotrope is used, whether an extender is utilised and the specific thixotrope. Generally, the thixotrope is used in a concentration of from about 1 to about 15% of the gel by weight, with approximately 4 to 8% being the preferred concentration. For instance, if a styrene-ethylene-propylene block co-polymer is used as the extender, the preferred concentration of the thixotrope is about 5%, but concentrations of from about 1 to about 8% have been used to advantage. If microspheres of either low density polyethylene, high density polyethylene or ethylene vinyl acetate co-polymer are utilised as the thixotrope in, for instance, a dielectric oil gel matrix, the preferred concentration is about 10% by weight. However, such thixotropes have been used in concentrations ranging from about 5 up to about 15% successfully. If a petroleum hydrocarbon of, for instance, aliphatic or napthenic paraffins, or a mixture of the two paraffins, is used as a gel matrix, the amount of thixotrope added ranges from about 5 to about 10%. When silica is used as a thixotrope with such oils, the concentration used is between about 4 and about 8%, the preferred concentration being about 6%. The thixotrope is used to prepare gel compositions with desired viscosities of from about 1.2 to about 1.8 million centipoises at 25°C. To counteract the tendency of the oil component of the filled cable of the present invention to swell the insulation around the conductors and migrate into the cells, it is preferred that oils of relatively high molecular weight must be utilised. Preferably oils having a molecular weight of between about 400 and about 1000 are utilised; if the dielectric gel matrix is formulated with an extender such as a microcrystalline polyethylene block co-polymer, or thermoplastic rubber, oils having a molecular weight as low as about 200 may also be used to advantage to give the desired viscosity. Such oils are available from, for instance, Penreco Corp. (i.e.. N1500 HT, having a molecular weight of about 520), Shell Corporation (available in several molecular weights, including a molecular weight of 400 which has been utilised to advantage), Amoco (e.g. Amoco 31 having a molecular weight of 400), Exxon (TUFFLO 30 (molecular weight of 460 ± 10))and TUFFLO 50 (molecular weight of weight of 370 ± 10). In addition, a corrosion inhibitor and antioxidant are added to the composition of the present invention. Suitable corrosion inhibitors include certain corrosive inhibitors which are typically used in greases which were found to have no effect on the water absorbency or insulation characteristics of the polymer of the gel composition. The rust inhibitor(s) must be chosen with care because those which are of acid character may neutralise the effect of the polymer. Preferred inhibitors are those sold under the REOMET (Ciba-Geigy Corp.) trademark such as REOMET 39 LF. A neutral barium dinonylnaphthalene sulfonate did not affect the properties of the present invention but did have a slight tendency to de-gel one of the gel compositions. A copper passivator which is a liquid copper triazole derivative was used without any adverse affects. Many antioxidants are known in the industry; particularly preferred are the antioxidants sold under the trademark IRGANOX (Ciba-Geigy Corp.), but it is not intended that the scope of the invention be restricted only to compositions including that particular antioxidant. The cable of the present invention is made with conventional cable filling equipment in the manner in which that equipment is utilised to make cable filled with PEPJs and/or ETPRs. Unlike known prior processes for making ETPR and/or PERJ-filled cable, however, the process is conducted at ambient temperature. The process involves drawing pairs of wires through a chamber and die in which the gel composition is extruded onto the wires, passing the wires through a sizing insert and then applying a wrapping to the bundle of coated wires. The wrapped bundle (or several wrapped bundles) is then drawn through a second die in which a second layer of the gel composition is extruded onto the bundles if desired and a sheath or jacket is then applied. The following are examples of different combinations of gel matrices and mixtures which thicken to produce a gel matrix which is appropriate for use with the water absorbent polymer having pendent anionic groups. The examples of compositions prepared in accordance with the invention are not intended to limit the scope of the invention and are instead illustrative of a number of different compositions which can be used to practice the invention. EXAMPLE 1 In a presently preferred embodiment a composition for filling a cable in accordance with the present invention is made in accordance with the present invention is made in accordance with the following formula (all percentages by weight): base oil (N-1500 HT, Penreco) 79.2% thixotrope thermoplastic rubber (KRATON G1701X, Shell Chemicals Corp.) 1.5% fumed silica (Cab-O-Sil TS720) 2.0% and Cab-O-Sihl M-5, Cabot) 4.0% polymer (FAVOR 960, Chemische Fabrik Stockhausen GmbH) 12.0% antioxidant (IRGANOX 1035, Ciba-Geigy Corp.) 1.3% In various formulations, these ingredients have been varied in the following proportions: base oil 65-82% thixotrope (in various combinations of silica, thermoplastic rubber, and organophilic clay) 6-14% polymer 19-20% antioxidant 0.05-1.5% In a particularly preferred embodiment, the polymer is dispersed in the gel matrix,and the gel thickened, by use of hydraulic shear force using a colloid mill in a continuous process for producing the composition. The composition is then used for filling a cable in the manner described above. EXAMPLE 2 The ability of the filled cable of the present invention to mitigate temperature related changes in capacitance is further illustrated by the following data. An ATG was formulated from the same components and in the same proportions ass set out in Example 1 but substituting a FAVOR 944 water absorbing polymer for the FAVOR 960 polymer and utilising the colloid mill for thickening of the gel. Using 25°C as the "zero" point, the per cent change in the dielectric constant of the gel was then measured and calculated as follows: Temperature (°C) % Change in Dielectric Constant -3 -5.15 14 -2.20 25 --- 35 2.59 45 4.26 55 5.18 60 5.69 EXAMPLE 3 Electrical performance of the cable of the present invention was tested in the following manner. In a preliminary experiment, twisted pairs of 26 gauge foam-skin insulated wires were immersed in beakers containing the composition made in accordance with Example 1 and two commercially available ETPRs at 60°C and capacitance was measured as a function of time. The results are shown in Figure 1. As can be seen, the ATG of the present invention was more stable than the ETPRs. The capacitance of the blue-white pairs of two 25 pair cables with 24 gauge solid insulation, one cable filled with the ATG of Example 1 and the other with a commercially available ETPR, were measured as a function of temperature. The data is shown in Figure 2. As can be seen, the capacitance of the ETPR filled cable decreases with increasing temperature while that of the ATG-filled cable remains relatively level. The relative stability of the ATG filled cable appears to result from the selection of the water absorbent polymer, the thixotrope, or a combination of polymer and thixotrope, which, when used to make the ATG of the present invention, results in an increase in the dielectric constant of the ATG. The temperature-dependent increase in the dielectric constant of the ATG mitigates the decrease in the capacitance of the cable resulting from thermally-induced expansion of the spaces between the wires of the cable. In a preferred embodiment, the polymer, thixotrope, or the combination of thixotrope and polymer, are selected so as to cause an increase in the dielectric constant of the ATG by about 4 to about 6% depending upon the specific polymer and polymer concentration, over a temperature range of about 40°C. EXAMPLE 4 The ability of the cable of the present invention to meet and/or exceed current requirements for cable electricals was demonstrated in the following manner. Table I contains the electrical data for the 50 pair super-unit of a APMW 200 cable containing an ATG filling compound manufactured in accordance with Example 1, above. These data are typical of filled cable and fit comfortably within the range required by industry Specifications. The value of R/C ratio for the cable was 0.0173 (0.0200 is passing). Average Std. Dev. Ind. Max. Ind. Min. mutual cap. (nF/mile)nF/km) (83.53) 51.90 (1.37) 0.85 (86.30) 53.62 (80.99) 50.32 attenuation (dB/Kf)dB/km (5.30) 17.39 (0.04) 0.13 (5.41) 17.75 (5.21) 17.09 EXAMPLE 5 The resistance of the insulation of the cable of the present invention to the oxidative cracking described above is demonstrated by the following data. Cables filled with the ATG composition of Example 1 were tested for oxidation by aging for various periods and measuring oxidation induction time (OIT). The test instrument was a Perkin-Elmer DSC4 using the Model 3700 Data Station. The test procedure was per Bellcore Specification TA-NWT-000421. In the first test, a 50 pair 24 gauge foam-skin cable filled with an ATG made in accordance with Example 1 was aged for four weeks at 70°C. OIT was determined at 200°C before and after aging. The type of stabilisation package used in the insulation was not known, nor was the cable's prior thermal history. In the second test, three cables, one filled with ATG made in accordance with Example 1 arid two with different ETPRs, were evaluated at 200°C after aging two weeks at 70°C. All three cables were 50 pair, 26 gauge foam-skin insulated conductors. The insulated conductors in all three cables were from the same wire production run. Again, the stabilisation packages were not known. The data for both tests are shown in Table II. All values reported represent the average of one run on each of the ten colour insulations. Oxidative Induction Time of Foam-Skin Insulation @ 200° C Induction Time, min. @ 200° C Test #1 4 Weeks Aging @ 70°C Test #2 2 Weeks Aging @ 70°C Filling Compound Av. Std. Dev. Av. Std. Dev. ATG 53.7 7.4 64.1 4.6 ETPR I - - 53.5 8.7 ETPR II - - 46.3 6.3 Although the invention has been described in terms of a number of examples setting forth preferred embodiments thereof, those skilled in the art who have the benefit of this disclosure will recognise that changes may be made in the compositions described in these various examples without changing the manner in which the various components of the gel composition of the present invention function to accomplish the results achieved by these compositions. Such changes might, for instance, take the form of small variations in the proportions of the various components, the substitution of some substance having a similar function not mentioned in the Specification for one of the components of the get composition, or the addition of a substance to one of the gel compositions described.

Claims: A telecommunications cable comprising a plurality of paired conductors, insulation surrounding said paired conductors, and a wrapping round the insulated, paired conductors, the space therebetween being filled with a composition characterised in that the cable has a substantially stable mutual capacitance between said paired conductors as the cable undergoes thermal expansion, said composition being comprised of a dielectric base oil, a water absorbing polymer, and a thixotrope, wherein the said components are selected such that the composition has a dielectric constant which increases as the temperature to which the cable is exposed increases, thereby mitigating the decrease in the mutual capacitance of the cable resulting from the expansion effect of increased temperature on the paired conductors, insulation and wrapping. The cable of Claim 1, wherein the water absorbing polymer is in the range of about 5 to about 33 weight percent of the composition. The cable of Claim 1 or 2, wherein the dielectric oil is in the range of about 65 to about 82 weight percent of the composition. The cable of Claim 1, 2 or 3, wherein the thixotrope is in the range of about 1 to about 15 percent of the composition. The cable of Claim 4, wherein the thixotrope is silica and is in the range of about 4 to about 8 percent of the composition. The telecommunications cable of Claim 1, wherein the composition is formed by mixing about 70 to 85 weight percent dielectric base oil, about 1 to 15 weight percent thixotrope, and about 6 to 20 weight percent water absorbing polymer. The cable of any one of the preceding Claims, wherein the dielectric oil is an oil having a molecular weight in the range of 400 to about 1000. The telecommunications cable of any one of Claims 1 to 6 wherein the dielectric base oil is comprised of a hydrocarbon oil having a molecular weight of about 200 or higher and an extender. The telecommunications cable of Claim 8 wherein the extender is a block co-polymer, thermoplastic rubber, polyethylene powder, microcrystalline wax, or polyethylene or thylene microspheres, or mixtures of same. A method of making a telecommunications cable comprising the steps of wrapping a plurality of paired, insulated conductors into a bundle, and filling the spaces between the paired, insulated conductors in the bundle with a composition characterised in that the composition is comprised of a dielectric base oil, a water absorbing polymer, and a thixotrope, wherein the said components are selected such that the composition has a dielectric constant which increases as the temperature to which the cable is exposed increases, thereby mitigating the decrease in the mutual capacitance of the cable resulting from the expansion effect of increased temperature on the paired conductors, insulation and wrapping. The method of Claim 10 wherein a plurality of bundles of paired, insulated conductors are bound together with a sheath, and wherein the spaces inside the sheath and between the bundles are also filled with the composition.

Inventors: DAVIS LAWRENCE EUGENE, FREEMAN CLARENCE S, DAVIS, LAWRENCE EUGENE, FREEMAN, CLARENCE S.
Applicants: WATERGUARD IND INC, WATERGUARD INDUSTRIES, INC.
Application year: 1994
Main classification: H01B7
IPC codes: H01B7, H01B3